This relates generally to phase change memories.
As used herein, a phase change memory is a device which uses a chalcogenide material as a memory element. A memory element is the element that actually stores information. Thus, as used herein, a phase change memory stores information on a memory element by changing the phase of the element between amorphous and crystalline phases. Generally, the amorphous phase is associated with what is called a reset state and the crystalline state is associated with what is called a set state. Multilevel memories may have a number of states intermediate between set and reset states.
In the programming operation, a phase change memory may be transitioned through the application of bias voltages and resulting currents to transition from amorphous to crystalline states or from crystalline to amorphous states. Generally, in these transitions, it is desirable to only change the phase of the cells you want to change and not the cells that you do not want to change.
Phase change memories are arranged in what is called a cross point memory where address lines are arranged in a grid. These address lines, normally called the bitline and word line, cross and memory elements span between the word lines and the bitlines. Generally, to select a memory element, a select device is added to each memory cell. That is, a memory cell may include a series connected select device and memory element.
In some phase change memories, an ovonic threshold switch is used as the select device. The ovonic threshold switch includes a chalcogenide material that does not change phase. Thus, the ovonic threshold switch does not meet the definition of a memory element set forth above.